Firmware update 7.7.8 is recommended for all AirPort Extreme and AirPort Time Capsule base stations with 802.11ac. This update addresses an issue that may prevent AirPort base stations from appearing in AirPort Utility when Back to My Mac is enabled.
About AirPort Base Station Firmware Update 7.6.8
Firmware update 7.6.8 is recommended for all Apple 802.11n Wi-Fi base stations including AirPort Express, AirPort Extreme and AirPort Time Capsule. This update addresses an issue that may prevent users from accessing their AirPort base station using Back to My Mac.
Apple periodically offers software updates (called firmware updates) that improve AirPort performance and functionality. Here's how to keep your base station up to date.
How to get the latest firmware for your AirPort base station...